Useful hot air: how compressed air power storage works

We take a look at how compressed air power storage operates in practice, …

Recently, we discussed the prospect of using compressed air as a form of energy storage. Unfortunately, it's hard to get much in the way of details on how the process actually works; there's only a single operational facility in the world actually using compressed air as a form of energy storage, and it's located in Alabama. We were lucky enough to see a talk by Roy Daniels of Energy Storage and Power, a company formed to bring the second generation of plants online, and gleaned more details about the new technology.

Energy Storage and Power is a collaboration between a major utility (PSE&G) and the companies that hold the patents involved in compressed air storage. Their aim is to license the technology for installations that can range in scale from 15MW to 500MW.

Plants using compressed air power can use the compressors to store off-peak power from renewable sources, and they have a number of additional advantages. Since compressed air power ramps quickly, it can be used to smooth the grid's frequency and handle sudden surges of load, something that's not possible with a number of other power sources.

Despite all its advantages, it works best when paired with a traditional combustion turbine plant. Although these could potentially burn some form of biofuel, realistically this will mean fossil fuels for the foreseeable future. Combustion turbines rely on high-temperature, high-pressure air to drive generators. Once through the turbine, the air is typically low-pressure, but it retains much of the heat of combustion.

This heated air is a great complement for compressed air, which is high-pressure but low temperature. By combining the two, heat that would generally be lost as waste can be combined with the compressed air to drive turbines. According to Daniels, for every 100MW of standard power, the compressed air can provide an additional 200MW of power. He also noted that, because it doesn't rely on steam, these plants can easily be operated in areas facing water shortages, which is already a problem throughout many western states.

Since all the technology involved is well-understood, there are few barriers to obtaining permits. Smaller plants can get sufficient gains with above-ground pressure tanks, but larger facilities need to be placed near an underground formation that can store the compressed air. Daniels argued that it's possible (and might make sense) to simply move the equipment from existing plants to locations with the right geology.

The returns on power aren't brilliant; each megawatt of power converted into compressed air only returns 0.67MW when released. But the fact that it allows useful power to be drawn from waste heat could up the effective yield to about 90 percent. This is one of those cases where green tech may face a no-free-lunch situation, as a number of technologies may wind up competing to harvest waste heat.

Ultimately, however, the biggest long-term issue might be the fact that this technology still involves burning fossil fuels. It may be possible to integrate with some forms of renewable power that also produce waste heat, such as solar thermal, but then having a combination of appropriate geology and weather would become essential. Still, given its status as proven on a utility-scale project, it's in better shape than most other potential bridge technologies.